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Formation mechanism of nano-hardystonite powder prepared by mechanochemical synthesis
Advanced Powder Technology xxx (2016) xxx–xxx
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Formation mechanism of nano-hardystonite powder prepared by mechanochemical synthesis Sorour Sadeghzade ⇑, Rahmatollah Emadi, Sheyda Labbaf Biomaterials Research Group, Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 9 March 2016 Received in revised form 18 July 2016 Accepted 27 August 2016 Available online xxxx Keywords: Bio-ceramic Hardystonite Nano-powders Mechanochemical synthesis
a b s t r a c t Hardystonite is currently recognized as a biocompatible bio-ceramic material for a range of medical applications. In this study, pure nano-crystalline hardystonite powder was prepared by mechanochemical synthesis of calcium carbonate, zinc oxide and silicate oxide in a planetary ball mill followed by sintering. A range of techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were applied to fully characterize the obtained powders. The effect of time and sintering temperature on the formation mechanism of nano-hardystonite was studied. It was found that pure nano-crystalline hardystonite powder formation occurred following 20 h of milling and subsequent sintering at 900 °C for 3 h. The measured crystallite and agglomerate particle size were found to be 28 ± 2 and 191 ± 3 nm, respectively. The two-step sintering processing was also applied for the preparation of bulk hardystonite. The compressive strength and elastic modulus of bulk hardystonite with 75.5 ± 3% relative density were approximately 121 ± 2 MPa and 27 ± 4 GPa, respectively. Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Ceramic is a non-metallic, inorganic material that have a wide range of applications in aerospace, refinery, chemical industries, electronics, optics and healthcare [1,2]. Amongst these applications, much interest has been devoted to the use of ceramics and glasses in biomedical applications including soft and hard tissue replacement and regeneration [3]. The great potential applications of bio-ceramics are due to their high biocompatibility, bioactivity, easy fabrication and high stiffness [4]. However, regardless of their suitable biological properties, the inherent brittle nature of this class of material limits their application in medicine [5]. Often, bio-ceramics are used with polymeric matrix to produce biocomposites [6] or are coated on the surface of implants to enhance tissue-material interactions in vivo [7]. Hydroxyapatite is an example of a bio-ceramic that has been in use for the past few decades as bone substituted material [8]. However, the weak mechanical properties (unsuitable elastic modulus, low fracture toughness and brittle behavior) and the low degradation rate of hydroxyapatite in biological environment limits their use in tissue engineering applications [9]. Recently, Calcium silicates (e.g. CaSiO3) have received great attention as an alternative ⇑ Corresponding author. Fax: +98 (031) 33912752. E-mail address: [email protected] (S. Sadeghzade).
bio-ceramic for hard tissue engineering due to their enhanced mechanical properties compared to hydroxyapatite [10]. However, calcium silicates are known to have a high degradation rate in physiological environment, leading to an increase in the local pH of the surrounding tissue, hence resulting in tissue damage [11]. The addition of elements such as Zn, Mg and Zr as network modifier into calcium silicate network can improve the dissolution and degradation rate [12]. Previous studies on hardystonite (Ca2ZnSi2O7) have demonstrated good mechanical [13] and biological properties. The mechanical properties of hardystonite, such as bending strength (136 MPa), fracture toughness (1.24 MPa m1/2) and Young’s modulus (37 GPa) are close to that of bone [13]. This is considered advantageous since for bone tissue replacement and regeneration the stress-shielding phenomenon, which may occur due to the difference mechanical properties of replaced material with natural tissue, can be negligible [14]. In addition, hardystonite possess enhanced chemical stability, in physiological environment, compared to calcium silicates [15]. The release of Zn from hardystonite network has shown to have anti-inflammatory and anti-bacterial properties [16]. To broaden the medical applications of hardystonite, Wang et al. [17] reported the fabrication of hardystonite-wollastonite scaffolds with a higher compressive strength and lower degradation rate compared to pure wollastonite scaffolds. In another study, hardystonite was successfully coated on Ti-6Al-4V substrate by
http://dx.doi.org/10.1016/j.apt.2016.08.010 0921-8831/Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Please cite this article in press as: S. Sadeghzade et al., Formation mechanism of nano-hardystonite powder prepared by mechanochemical synthesis, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.08.010
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plasma spraying method and an improved cell attachment, proliferation and differentiation were observed [18]. Zeriqat et al. [19] reported that the incorporation of Zn and Sr in calcium silicate ceramics could improve bone tissue regeneration and integration when tested in rat tibia in vivo. Wu et al. [13] produced hardystonite powder through sol-gel synthesis route and applied a high sintering temperature (1200 °C) during the processing, which resulted in irregular, agglomerated and large (5–40 lm) particles. Therefore, a reduction in sintering temperature is a step to achieve nanostructures with enhanced surface properties. Recently, mechanochemical synthesis has proven to be an effective and economical method for nanostructure and nano-crystalline ceramic fabrications. In this method, surface of the reactants are mechanically activated and so a lower processing temperature is often required [20]. The main objective of this study was to obtain pure nano-hardystonite by mechanochemical synthesis and subsequent sintering. Phase evaluation during processing and the chemical reactions of nanohardystonite formation are also studied. The effects of ball milling time and sintering temperature on the formation mechanism of nano-hardystonite are also evaluated. In addition, the two-step sintering method is applied for the first time to prepare bulk nano-hardystonite ceramics. 2. Materials and methods 2.1. Powder preparation by MA method In this research, hardystonite powder was prepared by mechanochemical synthesis. zinc oxide (ZnO, 99% purity, Merck), calcium carbonate (CaCO3, 98% purity, Merck) and silicate oxide (SiO2, 99%purity, Aldrich) powders, with a molar ratio of 1:2:2 respectively, were mixed in a planetary ball mill (Retsch, PM 100) in zirconia vial containing five zirconia balls of 20 mm in diameter. The ball/powder mass ratio was 10:1 and the rotational speed of the disc and vial was set at 250 and 500 rpm, respectively. The time of milling was chosen at 5 min and 2 h, 5 h, 10 h, 20 h followed by sintering at 900 °C, 1000 °C and 1100 °C for 3 h. The complete details of each sample are presented in Table 1.
Philips) equipped with Cu Ka radiation (k = 0.154 nm at 40 kV and 30 mA). Data were collected over the 2h range of 20–70° by scan rate of 0.05° per second. The crystallite size was measured by Scherrer equation (Eq. (1)). Three peaks with high intensity were selected for calculation of crystallite sizes of powder after subsequent sintering [21].
b cos h ¼
kk D
ð1Þ
where h is the Bragg diffraction angle, D is the crystallite size, k is the wavelength of the X-ray radiation, b is the diffraction peak width at half maximum intensity and K is a constant related to crystallite shape, normally taken as 0.9. Particle size and morphology were studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) using Philips EM208S with 100 kV operating voltage and Philips XL30 was working by acceleration voltage of 30 kV, respectively. The particle sizes of milled powders were estimated from both TEM and SEM images by using Image analysis software (image J). The milled powders were uniaxial pressed into pellets of 12 mm 18 mm under pressure of 600 MPa. The samples were then sintered at 1100 °C for 30 min and 900 °C for 13 h, respectively. The compressive strength and elastic modulus were measured by Hounsfield (H25KS). In this study, the liner shrinkage of hardystonite and its relative density (by Archimedes technique) were evaluated according to the following equations:
Liner Shirinage ¼
Lg Ls Lg
Relativ e density ¼
qm qt
ð2Þ ð3Þ
where Lg is the bulk length before sintering, Ls is the bulk length after sintering, qm is the measured density and qt is the theoretical density. Five consequent samples were used for each mechanical and physical test. 3. Results and discussion
2.2. Particle characterization
3.1. X-ray diffraction evaluation and reaction mechanism
Phase transformation evaluations for the powders prepared by mechanochemical synthesis at different time and sintering temperature were studied using X-ray diffraction (XRD) (X’pert
Fig. 1 represents the XRD pattern of H1–H5 samples. Fig. 1 (H1) shows the XRD patterns for the raw materials, which are consistent with the standards for CaCO3 (XRD data file No. 5-0586), SiO2 (XRD data file No. 46-1045) and ZnO (XRD data file No. 1-075-0576) compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS). Milling for 20 h (H5) has led to a broadening of the XRD peaks and a significant decrease in their intensities because of the internal strain and the formation of nano-crystalline materials during Mechanochemical synthesis. Also this phenomenon can be due to gradual transfer of the raw materials to amorphous state. According to Fig. 1 new crystallite phase at various milling time was not detected. Hence it can be suggested that hardystonite and other intermediate phases are unable to form just by applying mechanochemical method. The XRD patterns for H6–H10 samples are presented in Fig. 2. All the samples were sintered at 900 °C. The XRD pattern for sample after 5 min ball mill followed by sintering at 900 °C is presented in Fig. 2 (H6) where sharp peaks for CaO (XRD JCPDS data file No. 17-0912) are detected. Le and Oh [22] suggested that completion of reaction (4) occurs at 800 °C. Results presented in Figs. 1 (H1) and 2 (H6) suggest that the first step of hardystonite formation might have occurred based on the following Eq. (4):
Table 1 Sample conditions for mechanochemical method. Samples
Temperature (°C)
MA time (h)
H1 H2 H3 H4 H5 H6 H7 H8 H9 H10 H11 H12 H13 H14 H15 H16 H17
– – – – – 900 900 900 900 900 1000 1000 1000 1000 1000 1100 1100
5 min 2 5 10 20 5 min 2 5 10 20 5 min 2 5 10 20 10 20
Please cite this article in press as: S. Sadeghzade et al., Formation mechanism of nano-hardystonite powder prepared by mechanochemical synthesis, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.08.010
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Fig. 1. X-ray diffraction patterns of the prepared powders at different milling times.
Fig. 2. X-ray diffraction patterns of the prepared powders at different milling times followed by sintering at 900 °C for 3 h.
CaCO3 ! CaO þ CO2
ð4Þ
It is clear that the fabrication of hardystonite may not occur directly during different time periods of milling. Wu et al. [13] reported the presence of willemite intermediate phases, as byproduct, during the formation of hardystonite through sol-gel method. The formation of zinc silicate components, particularly willemite and zinc silicate phases are inevitable during the preparation of hardystonite by mechanochemical method. The zinc silicate (XRD JCPDS data file No. 1-070-0852) peaks were detected (Fig. 2 (H6)), which may have formed according to the following reaction (5):
ZnO þ SiO2 ! ZnSiO3
ð5Þ
ZnSiO3 is an unstable phase during annealing procedure, thus it is suspected that some zinc silicate that are found (Fig. 2 (H6)) had transferred to hardystonite or perhaps willemite based on the following reactions (6) and (7).
ZnSiO3 þ 2CaO þ SiO2 ! Ca2 ZnSi2 O7
ð6Þ
ZnSiO3 þ ZnO ! Zn2 SiO4
ð7Þ
The XRD pattern for H7 sample (Table 1) shows a strong peaks of willemite (Zn2SiO4) (XRD JCPDS data file No. 1-083-2270) and low intensity peaks of hardystonite (XRD JCPDS data file No. 1075-0916). Sintering of H8 sample after 5 h milling led to complete disappearance of ZnO peaks and a strong hardystonite peak. Traces of CaO, SiO2 and willemite were also observed at 5 h milling and subsequent sintering at 900 °C. The presence of these phases after 5 h milling shows the lack of powder homogeneity and long diffusion path. In the XRD patterns (Fig. 2 (H9)) for 10 h milled powders followed by sintering at 900 °C, the characteristic peaks of willemite and hardystonite can be observed. Based on Reference Intensity Ratio (RIR) method [23] (in X’pert highscore plus software) the amount of willemite is approximately 13 wt%. Hardystonite phase
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could only be detected after 20 h MA as shown in Fig. 2 (H10). It is suggested that the formation of hardystonite occurs by the following reaction:
ZnO þ 2SiO2 þ 2CaO ! Ca2 ZnSi2 O7
ð8Þ
However, based on the XRD patterns (Fig. 2) and the presence of willemite intermediate phase, it is assumed that the reactions can occur in various stages based on the following reactions (9) and (10):
2ZnO þ SiO2 ! Zn2 SiO4 Zn2 SiO4 þ 3SiO2 þ 4CaO ! 2Ca2 ZnSi2 O7
ð9Þ ð10Þ
Willemite is a member of zinc silica system. Zhang et al. [24] fabricated willemite bio-ceramic with a bending strength of 91 MPa, which is lower than that of hardystonite (136 MPa). However, both willemite and hardystonite show the same elastic modules close to natural bone [13,24]. However, the absence of willemite can improve the mechanical properties of hardystonite. As it can be observed, by increasing the temperature and milling time it is feasible to drive willemite to hardystonite. According to the reactions (8)–(10) almost all ZnO is consumed for the preparation of willemite and hardystonite. From XRD patterns (Figs. 2 and 3) the ZnO peaks disappeared under 5 h milling. Therefore, it is anticipated that reactions (8)–(10) occur before 5 h and it seems that reaction (10) may have occurred after 5 h. The low intensity peaks of hardystonite detected (Fig. 2 (H6)) after 5 min MA and subsequent sintering at 900 °C, indicate nucleation of hardystonite phase. Noticeably utilization of ball mill can enhance the kinetics of hardystonite formation because of welding, fracturing and re-welding of particles, which subsequently results in particle size reduction [20]. Also, a reduction in diffusion path is another main reason for facilitating kinetics of reactions. Furthermore, at nano-scale the material’s surface is the main contributing factor in many of the particle properties, due to high surface energy and increased surface area to volume ratio of nanoparticles. By considering a spherical particle, the equation of partial chemical potential for a given particle is calculated based on GibbsThomson equation [25]:
lri ¼ li þ
2c Vi r
ð11Þ
where lri is the surface chemical potential in a particle, r is the radius, li is surface chemical potential of flat surface, c is surface free energy and Vi is molecular volume. From Eq. (11) it is suggested that by increasing the surface free energy and decreasing the particle size results in increased chemical potential and hence shows a higher affinity for hardystonite formation. In Fig. 3, the samples milled for different times sintered at 1000 °C are presented. In the XRD pattern for H11 sample (Fig. 3 (H11)) more peaks of hardystonite with sharp peaks of the starting materials compared to Fig. 2 (H6) and some peaks of zinc silicate were observed. By increasing the milling time to 2 h and subsequent sintering at 1000 °C (Fig. 3 (H12)), the sharp peaks of hardystonite appeared in XRD but some peaks of CaO, SiO2, ZnO and willemite were also detected. Furthermore, the peaks of ZnO disappeared in the H13 XRD pattern. With increasing the milling time to 10 h (H14), the peak intensities of willemite reduced, also SiO2 and CaO peaks completely disappeared. As it can be seen in XRD pattern of H15 (Fig. 3 (H15)), only characteristic peaks of hardystonite are seen. Therefore after 20 h milling and increasing the temperature to 1000 °C (H15), the formation of hardystonite powder is completed. Fig. 4 shows the XRD patterns for powders after various milling time followed by sintering at 1100 °C. The sharp peaks of pure hardystonite were observed in H16 and H17 XRD pattern. Because of the formation of this powder without any impurity only by 10 h milling (H16), it seems that by increasing the temperature to 1100 °C improves the kinetics of hardystonite formation. However, the grain growth is considered as a negative effect of increasing temperature. Fig. 4 (H17) shows a reduction in peak intensity of hardystonite after 20 h milling, which may be due to increase in the milling time. The crystallite size of hardystonite powder was measured by Scherrer equation for powders prepared at different conditions (Fig. 5). Both milling time and sintering temperature were found to have an effect on crystallite size. Overall, increase in milling time was found to reduce the crystallite size. In fact during milling time the particles get fragmented and this in turn results in reduction in
Fig. 3. X-ray diffraction patterns of the prepared powders at different milling times followed by sintering at 1000 °C for 3 h.
Please cite this article in press as: S. Sadeghzade et al., Formation mechanism of nano-hardystonite powder prepared by mechanochemical synthesis, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.08.010
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Fig. 4. X-ray diffraction patterns of the prepared powders at different milling times followed by sintering at 1100 °C for 3 h.
Fig. 5. Crystallite size of hardystonite powders as a function of milling times and sintering temperatures.
crystallites size. For example H16, which was milled for 10 h were larger (136 ± 4 nm) than H17 samples (111 ± 2 nm) that were milled for 20 h but at the same sintering temperature. The crystallite size of H10 and H15 were found to be 21 ± 3 and 32 ± 2 nm, respectively. Interestingly the crystallite sizes for H9, H14, and H16 samples that were milled for 10 h but had different sintering temperature were measured 29 ± 1, 61 ± 1 and 136 ± 4 nm respectively. Hence, an increase in the sintering temperature results in increased crystallite sizes which can be attributed to the negative effect of temperature on grain growth. 3.2. TEM and SEM analysis TEM was applied to investigate the morphology and grain size of pure hardystonite after 20 h milling and sintering at 900 °C for 3 h. Fig. 6 shows pure hardystonite with spherical morphology with low degree of agglomeration. The grain size, measured by Image J, was found to be 28 ± 2 nm. These results are in good agreement with crystallite size measured by the Scherrer equation. Fig. 7 shows the SEM micrographs of nano-hardystonite powder under different milling times followed by sintering at 900 °C for 3 h. Also, Fig. 8 shows the changes in particle size of this powder during various milling times but at subsequent sintering temperature of 900 °C. As shown in Figs. 7 and 8, increase in milling time
Fig. 6. TEM micrograph of pure hardystonite nano powder at 20 h milling followed by sintering at 900 °C for 3 h.
from 5 min (1.7 lm) to 5 h (3.2 lm) has led to an increase in the overall particles size. This could be due to the increase in the tendency of particles towards agglomeration, since smaller
Please cite this article in press as: S. Sadeghzade et al., Formation mechanism of nano-hardystonite powder prepared by mechanochemical synthesis, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.08.010
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Fig. 7. SEM micrographs of prepared powders after (a) 5 min, (b) 5 h, (c) 10 h and (d and e) 20 h milling and subsequent sintering at 900 °C for 3 h.
Fig. 8. Changes in the particle size at different milling times followed by sintering at 900 °C for 3 h.
particles have a greater surface area to volume ratio and a higher surface energy; hence, the welding occurs more frequently between the particles. Increase in the milling time to 10 h (Fig. 7c) has caused a decrease in the mean particle size to 0.726 lm, which could be as a result of constant fracturing of powder and ball/powder collision. By increasing the milling time to 20 h, the mean agglomerate particles size of nano-hardystonite was found to be 191 ± 3 nm. An increase in the milling time caused a change in the powder morphology from lamellar and needle like (Fig. 7a) to spherical shape (Fig. 7d and e). This change in powder morphology can be observed during the milling times of 5 h and 10 h. This can also be explained by the tendency of the system to reduce the internal energy, which may result in rounded shape particles. Therefore it can be suggested that a reduction in sintering temperature is a step to achieve nano-materials. Considering the high surface area to volume ratio of nano- materials, it is highly accepted that improved features such as high contact area, diffusion rates, surface reactivity and reduced sintering time or temperature with enhanced biological properties can be obtained [5].
Fig. 9. The regime of two-step sintering for the preparation of green body hardystonite.
Table 2 The compressive strength, elastic modulus, relative density and liner shrinkage properties of hardystonite. Compressive strength (MPa)
Elastic modulus (GPa)
Relative density (%)
Liner shrinkage (%)
121 ± 2
27 ± 4
75.5 ± 3
4.1 ± 0.3
4. Mechanical properties To investigate the mechanical properties, nano-powders were milled for 20 h and pressed at 600 MPa. All samples were then sintered according to Fig. 9, which represents the regime of optimised two-step sintering process for the preparation the nanohardystonite green bodies. The two-step sintering is an effective way of reducing the grain growth in the final stage of sintering. At first, the sample is heated to a high temperature (T1) and it is kept for a short time and then the temperature is reduced to a lower temperature (T2) and kept for a long time (time is material dependent). During the second stage of sintering, the sample density is improved without any grain growth [26]. Table 2 shows the mechanical and physical
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S. Sadeghzade et al. / Advanced Powder Technology xxx (2016) xxx–xxx Table 3 Mechanical properties of cortical and cancellous bone, hydroxyapatite (HA) bio-glass, hardystonite.
Cortical bone Cancellous bone Hydroxyapatite Bio-glass 45S5 Hardystonite
Compressive strength (MPa)
Elastic modulus (GPa)
Relative density (%)
Porosity (%)
Reference
130–180 4–12 0.2–4 308–509 500 121
7–30 0.1–0.5 0.12–1.1 42–81 35 27
87–95 –
5–13 30–90
[4,27] [4,27]
81–97 – 75.5
2.8–19.4 – 24.5
[27,28] [4,29] In this study
properties of nano-hardystonite of green bodies. The compressive strength and elastic modulus of hardystonite with 75.5% density were found to be 121 ± 2 MPa and 27 ± 4 GPa, respectively. Table 3 presents the mechanical properties of human cortical and cancellous bone and also the commercially available bioceramics to provide a comparison with hardystonite samples prepared in this study. Based on the literature, the compressive strength and elastic modulus of HA and Bio-glass are in the range of 308–509, 500 MPa and 42–81, 35 GPa, respectively. As it can be seen in Table 3, the mechanical properties of these bio-ceramics are significantly higher than that of bone tissue. Furthermore a biomaterials’ elastic modulus is one of the most important mechanical properties for hard tissue applications. If biomaterials show a higher elastic modulus compared to the native bone tissue, the stress shielding phenomena may occur [14]. As a result, using materials with elastic modulus close to that of human bone is desirable. Since hardystonite has great potential for bone regeneration applications, it is advantageous to have its compressive strength (121 MPa) and elastic modulus (27 GPa) close to cortical bone which has a compressive strength and elastic modulus in the range of 130–180 MPa and 7–30 GPa, respectively. Nano-hardystonite powders can be used as bone filler, added to a polymer matrix to produce a composite or coated on the surface of metallic implants to enhance surface bioactivity and reduce surface corrosion. In addition, it is suggested that the ionic dissolution products of hardystonite following degradation can have a profound effect on cellular behavior both in vitro and in vivo. However, more thorough study is required to confirm these phenomena.
5. Conclusion In this study, the application of mechanochemical synthesis followed by sintering resulted in the formation of pure nanohardystonite powder. Optimum processing conditions of the prepared hardystonite nano powder was found to be at a milling time of 20 h followed by sintering at 900 °C. The XRD patterns were utilized to study the chemical reactions of hardystonite formation. The crystallite and agglomerate particle size for the prepared powders under 20 h milling and subsequent sintering at 900 °C (H10) were measured at 28 ± 2 and 191 ± 3 nm, respectively. Increasing the milling time caused a reduction in size of the prepared powders. The bulk hardystonite ceramics were prepared by two-step sintering method with 75.5 ± 3% relative density. The compressive strength and elastic modulus of the bulk hardystonite were measured at 121 ± 2 MPa and 27 ± 4 GPa, respectively. The overall results observed in this study could be beneficial for hard tissue engineering applications.
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Formation mechanism of nano-hardystonite powder prepared by mechanochemical synthesis Sorour Sadeghzade ⇑, Rahmatollah Emadi, Sheyda Labbaf Biomaterials Research Group, Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 9 March 2016 Received in revised form 18 July 2016 Accepted 27 August 2016 Available online xxxx Keywords: Bio-ceramic Hardystonite Nano-powders Mechanochemical synthesis
a b s t r a c t Hardystonite is currently recognized as a biocompatible bio-ceramic material for a range of medical applications. In this study, pure nano-crystalline hardystonite powder was prepared by mechanochemical synthesis of calcium carbonate, zinc oxide and silicate oxide in a planetary ball mill followed by sintering. A range of techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were applied to fully characterize the obtained powders. The effect of time and sintering temperature on the formation mechanism of nano-hardystonite was studied. It was found that pure nano-crystalline hardystonite powder formation occurred following 20 h of milling and subsequent sintering at 900 °C for 3 h. The measured crystallite and agglomerate particle size were found to be 28 ± 2 and 191 ± 3 nm, respectively. The two-step sintering processing was also applied for the preparation of bulk hardystonite. The compressive strength and elastic modulus of bulk hardystonite with 75.5 ± 3% relative density were approximately 121 ± 2 MPa and 27 ± 4 GPa, respectively. Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Ceramic is a non-metallic, inorganic material that have a wide range of applications in aerospace, refinery, chemical industries, electronics, optics and healthcare [1,2]. Amongst these applications, much interest has been devoted to the use of ceramics and glasses in biomedical applications including soft and hard tissue replacement and regeneration [3]. The great potential applications of bio-ceramics are due to their high biocompatibility, bioactivity, easy fabrication and high stiffness [4]. However, regardless of their suitable biological properties, the inherent brittle nature of this class of material limits their application in medicine [5]. Often, bio-ceramics are used with polymeric matrix to produce biocomposites [6] or are coated on the surface of implants to enhance tissue-material interactions in vivo [7]. Hydroxyapatite is an example of a bio-ceramic that has been in use for the past few decades as bone substituted material [8]. However, the weak mechanical properties (unsuitable elastic modulus, low fracture toughness and brittle behavior) and the low degradation rate of hydroxyapatite in biological environment limits their use in tissue engineering applications [9]. Recently, Calcium silicates (e.g. CaSiO3) have received great attention as an alternative ⇑ Corresponding author. Fax: +98 (031) 33912752. E-mail address: [email protected] (S. Sadeghzade).
bio-ceramic for hard tissue engineering due to their enhanced mechanical properties compared to hydroxyapatite [10]. However, calcium silicates are known to have a high degradation rate in physiological environment, leading to an increase in the local pH of the surrounding tissue, hence resulting in tissue damage [11]. The addition of elements such as Zn, Mg and Zr as network modifier into calcium silicate network can improve the dissolution and degradation rate [12]. Previous studies on hardystonite (Ca2ZnSi2O7) have demonstrated good mechanical [13] and biological properties. The mechanical properties of hardystonite, such as bending strength (136 MPa), fracture toughness (1.24 MPa m1/2) and Young’s modulus (37 GPa) are close to that of bone [13]. This is considered advantageous since for bone tissue replacement and regeneration the stress-shielding phenomenon, which may occur due to the difference mechanical properties of replaced material with natural tissue, can be negligible [14]. In addition, hardystonite possess enhanced chemical stability, in physiological environment, compared to calcium silicates [15]. The release of Zn from hardystonite network has shown to have anti-inflammatory and anti-bacterial properties [16]. To broaden the medical applications of hardystonite, Wang et al. [17] reported the fabrication of hardystonite-wollastonite scaffolds with a higher compressive strength and lower degradation rate compared to pure wollastonite scaffolds. In another study, hardystonite was successfully coated on Ti-6Al-4V substrate by
http://dx.doi.org/10.1016/j.apt.2016.08.010 0921-8831/Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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plasma spraying method and an improved cell attachment, proliferation and differentiation were observed [18]. Zeriqat et al. [19] reported that the incorporation of Zn and Sr in calcium silicate ceramics could improve bone tissue regeneration and integration when tested in rat tibia in vivo. Wu et al. [13] produced hardystonite powder through sol-gel synthesis route and applied a high sintering temperature (1200 °C) during the processing, which resulted in irregular, agglomerated and large (5–40 lm) particles. Therefore, a reduction in sintering temperature is a step to achieve nanostructures with enhanced surface properties. Recently, mechanochemical synthesis has proven to be an effective and economical method for nanostructure and nano-crystalline ceramic fabrications. In this method, surface of the reactants are mechanically activated and so a lower processing temperature is often required [20]. The main objective of this study was to obtain pure nano-hardystonite by mechanochemical synthesis and subsequent sintering. Phase evaluation during processing and the chemical reactions of nanohardystonite formation are also studied. The effects of ball milling time and sintering temperature on the formation mechanism of nano-hardystonite are also evaluated. In addition, the two-step sintering method is applied for the first time to prepare bulk nano-hardystonite ceramics. 2. Materials and methods 2.1. Powder preparation by MA method In this research, hardystonite powder was prepared by mechanochemical synthesis. zinc oxide (ZnO, 99% purity, Merck), calcium carbonate (CaCO3, 98% purity, Merck) and silicate oxide (SiO2, 99%purity, Aldrich) powders, with a molar ratio of 1:2:2 respectively, were mixed in a planetary ball mill (Retsch, PM 100) in zirconia vial containing five zirconia balls of 20 mm in diameter. The ball/powder mass ratio was 10:1 and the rotational speed of the disc and vial was set at 250 and 500 rpm, respectively. The time of milling was chosen at 5 min and 2 h, 5 h, 10 h, 20 h followed by sintering at 900 °C, 1000 °C and 1100 °C for 3 h. The complete details of each sample are presented in Table 1.
Philips) equipped with Cu Ka radiation (k = 0.154 nm at 40 kV and 30 mA). Data were collected over the 2h range of 20–70° by scan rate of 0.05° per second. The crystallite size was measured by Scherrer equation (Eq. (1)). Three peaks with high intensity were selected for calculation of crystallite sizes of powder after subsequent sintering [21].
b cos h ¼
kk D
ð1Þ
where h is the Bragg diffraction angle, D is the crystallite size, k is the wavelength of the X-ray radiation, b is the diffraction peak width at half maximum intensity and K is a constant related to crystallite shape, normally taken as 0.9. Particle size and morphology were studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) using Philips EM208S with 100 kV operating voltage and Philips XL30 was working by acceleration voltage of 30 kV, respectively. The particle sizes of milled powders were estimated from both TEM and SEM images by using Image analysis software (image J). The milled powders were uniaxial pressed into pellets of 12 mm 18 mm under pressure of 600 MPa. The samples were then sintered at 1100 °C for 30 min and 900 °C for 13 h, respectively. The compressive strength and elastic modulus were measured by Hounsfield (H25KS). In this study, the liner shrinkage of hardystonite and its relative density (by Archimedes technique) were evaluated according to the following equations:
Liner Shirinage ¼
Lg Ls Lg
Relativ e density ¼
qm qt
ð2Þ ð3Þ
where Lg is the bulk length before sintering, Ls is the bulk length after sintering, qm is the measured density and qt is the theoretical density. Five consequent samples were used for each mechanical and physical test. 3. Results and discussion
2.2. Particle characterization
3.1. X-ray diffraction evaluation and reaction mechanism
Phase transformation evaluations for the powders prepared by mechanochemical synthesis at different time and sintering temperature were studied using X-ray diffraction (XRD) (X’pert
Fig. 1 represents the XRD pattern of H1–H5 samples. Fig. 1 (H1) shows the XRD patterns for the raw materials, which are consistent with the standards for CaCO3 (XRD data file No. 5-0586), SiO2 (XRD data file No. 46-1045) and ZnO (XRD data file No. 1-075-0576) compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS). Milling for 20 h (H5) has led to a broadening of the XRD peaks and a significant decrease in their intensities because of the internal strain and the formation of nano-crystalline materials during Mechanochemical synthesis. Also this phenomenon can be due to gradual transfer of the raw materials to amorphous state. According to Fig. 1 new crystallite phase at various milling time was not detected. Hence it can be suggested that hardystonite and other intermediate phases are unable to form just by applying mechanochemical method. The XRD patterns for H6–H10 samples are presented in Fig. 2. All the samples were sintered at 900 °C. The XRD pattern for sample after 5 min ball mill followed by sintering at 900 °C is presented in Fig. 2 (H6) where sharp peaks for CaO (XRD JCPDS data file No. 17-0912) are detected. Le and Oh [22] suggested that completion of reaction (4) occurs at 800 °C. Results presented in Figs. 1 (H1) and 2 (H6) suggest that the first step of hardystonite formation might have occurred based on the following Eq. (4):
Table 1 Sample conditions for mechanochemical method. Samples
Temperature (°C)
MA time (h)
H1 H2 H3 H4 H5 H6 H7 H8 H9 H10 H11 H12 H13 H14 H15 H16 H17
– – – – – 900 900 900 900 900 1000 1000 1000 1000 1000 1100 1100
5 min 2 5 10 20 5 min 2 5 10 20 5 min 2 5 10 20 10 20
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Fig. 1. X-ray diffraction patterns of the prepared powders at different milling times.
Fig. 2. X-ray diffraction patterns of the prepared powders at different milling times followed by sintering at 900 °C for 3 h.
CaCO3 ! CaO þ CO2
ð4Þ
It is clear that the fabrication of hardystonite may not occur directly during different time periods of milling. Wu et al. [13] reported the presence of willemite intermediate phases, as byproduct, during the formation of hardystonite through sol-gel method. The formation of zinc silicate components, particularly willemite and zinc silicate phases are inevitable during the preparation of hardystonite by mechanochemical method. The zinc silicate (XRD JCPDS data file No. 1-070-0852) peaks were detected (Fig. 2 (H6)), which may have formed according to the following reaction (5):
ZnO þ SiO2 ! ZnSiO3
ð5Þ
ZnSiO3 is an unstable phase during annealing procedure, thus it is suspected that some zinc silicate that are found (Fig. 2 (H6)) had transferred to hardystonite or perhaps willemite based on the following reactions (6) and (7).
ZnSiO3 þ 2CaO þ SiO2 ! Ca2 ZnSi2 O7
ð6Þ
ZnSiO3 þ ZnO ! Zn2 SiO4
ð7Þ
The XRD pattern for H7 sample (Table 1) shows a strong peaks of willemite (Zn2SiO4) (XRD JCPDS data file No. 1-083-2270) and low intensity peaks of hardystonite (XRD JCPDS data file No. 1075-0916). Sintering of H8 sample after 5 h milling led to complete disappearance of ZnO peaks and a strong hardystonite peak. Traces of CaO, SiO2 and willemite were also observed at 5 h milling and subsequent sintering at 900 °C. The presence of these phases after 5 h milling shows the lack of powder homogeneity and long diffusion path. In the XRD patterns (Fig. 2 (H9)) for 10 h milled powders followed by sintering at 900 °C, the characteristic peaks of willemite and hardystonite can be observed. Based on Reference Intensity Ratio (RIR) method [23] (in X’pert highscore plus software) the amount of willemite is approximately 13 wt%. Hardystonite phase
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could only be detected after 20 h MA as shown in Fig. 2 (H10). It is suggested that the formation of hardystonite occurs by the following reaction:
ZnO þ 2SiO2 þ 2CaO ! Ca2 ZnSi2 O7
ð8Þ
However, based on the XRD patterns (Fig. 2) and the presence of willemite intermediate phase, it is assumed that the reactions can occur in various stages based on the following reactions (9) and (10):
2ZnO þ SiO2 ! Zn2 SiO4 Zn2 SiO4 þ 3SiO2 þ 4CaO ! 2Ca2 ZnSi2 O7
ð9Þ ð10Þ
Willemite is a member of zinc silica system. Zhang et al. [24] fabricated willemite bio-ceramic with a bending strength of 91 MPa, which is lower than that of hardystonite (136 MPa). However, both willemite and hardystonite show the same elastic modules close to natural bone [13,24]. However, the absence of willemite can improve the mechanical properties of hardystonite. As it can be observed, by increasing the temperature and milling time it is feasible to drive willemite to hardystonite. According to the reactions (8)–(10) almost all ZnO is consumed for the preparation of willemite and hardystonite. From XRD patterns (Figs. 2 and 3) the ZnO peaks disappeared under 5 h milling. Therefore, it is anticipated that reactions (8)–(10) occur before 5 h and it seems that reaction (10) may have occurred after 5 h. The low intensity peaks of hardystonite detected (Fig. 2 (H6)) after 5 min MA and subsequent sintering at 900 °C, indicate nucleation of hardystonite phase. Noticeably utilization of ball mill can enhance the kinetics of hardystonite formation because of welding, fracturing and re-welding of particles, which subsequently results in particle size reduction [20]. Also, a reduction in diffusion path is another main reason for facilitating kinetics of reactions. Furthermore, at nano-scale the material’s surface is the main contributing factor in many of the particle properties, due to high surface energy and increased surface area to volume ratio of nanoparticles. By considering a spherical particle, the equation of partial chemical potential for a given particle is calculated based on GibbsThomson equation [25]:
lri ¼ li þ
2c Vi r
ð11Þ
where lri is the surface chemical potential in a particle, r is the radius, li is surface chemical potential of flat surface, c is surface free energy and Vi is molecular volume. From Eq. (11) it is suggested that by increasing the surface free energy and decreasing the particle size results in increased chemical potential and hence shows a higher affinity for hardystonite formation. In Fig. 3, the samples milled for different times sintered at 1000 °C are presented. In the XRD pattern for H11 sample (Fig. 3 (H11)) more peaks of hardystonite with sharp peaks of the starting materials compared to Fig. 2 (H6) and some peaks of zinc silicate were observed. By increasing the milling time to 2 h and subsequent sintering at 1000 °C (Fig. 3 (H12)), the sharp peaks of hardystonite appeared in XRD but some peaks of CaO, SiO2, ZnO and willemite were also detected. Furthermore, the peaks of ZnO disappeared in the H13 XRD pattern. With increasing the milling time to 10 h (H14), the peak intensities of willemite reduced, also SiO2 and CaO peaks completely disappeared. As it can be seen in XRD pattern of H15 (Fig. 3 (H15)), only characteristic peaks of hardystonite are seen. Therefore after 20 h milling and increasing the temperature to 1000 °C (H15), the formation of hardystonite powder is completed. Fig. 4 shows the XRD patterns for powders after various milling time followed by sintering at 1100 °C. The sharp peaks of pure hardystonite were observed in H16 and H17 XRD pattern. Because of the formation of this powder without any impurity only by 10 h milling (H16), it seems that by increasing the temperature to 1100 °C improves the kinetics of hardystonite formation. However, the grain growth is considered as a negative effect of increasing temperature. Fig. 4 (H17) shows a reduction in peak intensity of hardystonite after 20 h milling, which may be due to increase in the milling time. The crystallite size of hardystonite powder was measured by Scherrer equation for powders prepared at different conditions (Fig. 5). Both milling time and sintering temperature were found to have an effect on crystallite size. Overall, increase in milling time was found to reduce the crystallite size. In fact during milling time the particles get fragmented and this in turn results in reduction in
Fig. 3. X-ray diffraction patterns of the prepared powders at different milling times followed by sintering at 1000 °C for 3 h.
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Fig. 4. X-ray diffraction patterns of the prepared powders at different milling times followed by sintering at 1100 °C for 3 h.
Fig. 5. Crystallite size of hardystonite powders as a function of milling times and sintering temperatures.
crystallites size. For example H16, which was milled for 10 h were larger (136 ± 4 nm) than H17 samples (111 ± 2 nm) that were milled for 20 h but at the same sintering temperature. The crystallite size of H10 and H15 were found to be 21 ± 3 and 32 ± 2 nm, respectively. Interestingly the crystallite sizes for H9, H14, and H16 samples that were milled for 10 h but had different sintering temperature were measured 29 ± 1, 61 ± 1 and 136 ± 4 nm respectively. Hence, an increase in the sintering temperature results in increased crystallite sizes which can be attributed to the negative effect of temperature on grain growth. 3.2. TEM and SEM analysis TEM was applied to investigate the morphology and grain size of pure hardystonite after 20 h milling and sintering at 900 °C for 3 h. Fig. 6 shows pure hardystonite with spherical morphology with low degree of agglomeration. The grain size, measured by Image J, was found to be 28 ± 2 nm. These results are in good agreement with crystallite size measured by the Scherrer equation. Fig. 7 shows the SEM micrographs of nano-hardystonite powder under different milling times followed by sintering at 900 °C for 3 h. Also, Fig. 8 shows the changes in particle size of this powder during various milling times but at subsequent sintering temperature of 900 °C. As shown in Figs. 7 and 8, increase in milling time
Fig. 6. TEM micrograph of pure hardystonite nano powder at 20 h milling followed by sintering at 900 °C for 3 h.
from 5 min (1.7 lm) to 5 h (3.2 lm) has led to an increase in the overall particles size. This could be due to the increase in the tendency of particles towards agglomeration, since smaller
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Fig. 7. SEM micrographs of prepared powders after (a) 5 min, (b) 5 h, (c) 10 h and (d and e) 20 h milling and subsequent sintering at 900 °C for 3 h.
Fig. 8. Changes in the particle size at different milling times followed by sintering at 900 °C for 3 h.
particles have a greater surface area to volume ratio and a higher surface energy; hence, the welding occurs more frequently between the particles. Increase in the milling time to 10 h (Fig. 7c) has caused a decrease in the mean particle size to 0.726 lm, which could be as a result of constant fracturing of powder and ball/powder collision. By increasing the milling time to 20 h, the mean agglomerate particles size of nano-hardystonite was found to be 191 ± 3 nm. An increase in the milling time caused a change in the powder morphology from lamellar and needle like (Fig. 7a) to spherical shape (Fig. 7d and e). This change in powder morphology can be observed during the milling times of 5 h and 10 h. This can also be explained by the tendency of the system to reduce the internal energy, which may result in rounded shape particles. Therefore it can be suggested that a reduction in sintering temperature is a step to achieve nano-materials. Considering the high surface area to volume ratio of nano- materials, it is highly accepted that improved features such as high contact area, diffusion rates, surface reactivity and reduced sintering time or temperature with enhanced biological properties can be obtained [5].
Fig. 9. The regime of two-step sintering for the preparation of green body hardystonite.
Table 2 The compressive strength, elastic modulus, relative density and liner shrinkage properties of hardystonite. Compressive strength (MPa)
Elastic modulus (GPa)
Relative density (%)
Liner shrinkage (%)
121 ± 2
27 ± 4
75.5 ± 3
4.1 ± 0.3
4. Mechanical properties To investigate the mechanical properties, nano-powders were milled for 20 h and pressed at 600 MPa. All samples were then sintered according to Fig. 9, which represents the regime of optimised two-step sintering process for the preparation the nanohardystonite green bodies. The two-step sintering is an effective way of reducing the grain growth in the final stage of sintering. At first, the sample is heated to a high temperature (T1) and it is kept for a short time and then the temperature is reduced to a lower temperature (T2) and kept for a long time (time is material dependent). During the second stage of sintering, the sample density is improved without any grain growth [26]. Table 2 shows the mechanical and physical
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S. Sadeghzade et al. / Advanced Powder Technology xxx (2016) xxx–xxx Table 3 Mechanical properties of cortical and cancellous bone, hydroxyapatite (HA) bio-glass, hardystonite.
Cortical bone Cancellous bone Hydroxyapatite Bio-glass 45S5 Hardystonite
Compressive strength (MPa)
Elastic modulus (GPa)
Relative density (%)
Porosity (%)
Reference
130–180 4–12 0.2–4 308–509 500 121
7–30 0.1–0.5 0.12–1.1 42–81 35 27
87–95 –
5–13 30–90
[4,27] [4,27]
81–97 – 75.5
2.8–19.4 – 24.5
[27,28] [4,29] In this study
properties of nano-hardystonite of green bodies. The compressive strength and elastic modulus of hardystonite with 75.5% density were found to be 121 ± 2 MPa and 27 ± 4 GPa, respectively. Table 3 presents the mechanical properties of human cortical and cancellous bone and also the commercially available bioceramics to provide a comparison with hardystonite samples prepared in this study. Based on the literature, the compressive strength and elastic modulus of HA and Bio-glass are in the range of 308–509, 500 MPa and 42–81, 35 GPa, respectively. As it can be seen in Table 3, the mechanical properties of these bio-ceramics are significantly higher than that of bone tissue. Furthermore a biomaterials’ elastic modulus is one of the most important mechanical properties for hard tissue applications. If biomaterials show a higher elastic modulus compared to the native bone tissue, the stress shielding phenomena may occur [14]. As a result, using materials with elastic modulus close to that of human bone is desirable. Since hardystonite has great potential for bone regeneration applications, it is advantageous to have its compressive strength (121 MPa) and elastic modulus (27 GPa) close to cortical bone which has a compressive strength and elastic modulus in the range of 130–180 MPa and 7–30 GPa, respectively. Nano-hardystonite powders can be used as bone filler, added to a polymer matrix to produce a composite or coated on the surface of metallic implants to enhance surface bioactivity and reduce surface corrosion. In addition, it is suggested that the ionic dissolution products of hardystonite following degradation can have a profound effect on cellular behavior both in vitro and in vivo. However, more thorough study is required to confirm these phenomena.
5. Conclusion In this study, the application of mechanochemical synthesis followed by sintering resulted in the formation of pure nanohardystonite powder. Optimum processing conditions of the prepared hardystonite nano powder was found to be at a milling time of 20 h followed by sintering at 900 °C. The XRD patterns were utilized to study the chemical reactions of hardystonite formation. The crystallite and agglomerate particle size for the prepared powders under 20 h milling and subsequent sintering at 900 °C (H10) were measured at 28 ± 2 and 191 ± 3 nm, respectively. Increasing the milling time caused a reduction in size of the prepared powders. The bulk hardystonite ceramics were prepared by two-step sintering method with 75.5 ± 3% relative density. The compressive strength and elastic modulus of the bulk hardystonite were measured at 121 ± 2 MPa and 27 ± 4 GPa, respectively. The overall results observed in this study could be beneficial for hard tissue engineering applications.
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Please cite this article in press as: S. Sadeghzade et al., Formation mechanism of nano-hardystonite powder prepared by mechanochemical synthesis, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.08.010